The invention relates to an automatic focusing system for an inspection apparatus which is particularly adapted for the inspection of glass sheet. One of the problems with conventional auto focusing systems is that they do not inherently provide a reference for the sample surface. That is, most of the prior art auto focusing systems rely on maximizing the edge contrast of a particle or inclusion in glass which provides a sharp image of the individual particle. However, such focusing did not yield any information about the location of the particle relative to the surface of the glass. For surface or depth information, these known techniques often rely on a calibrated axis that is parallel to the optical axis, which requires the glass to be held in a fixed plane. Other techniques, utilizing laser, rely on reflecting rays of light off the glass surface. While these techniques do compensate for variations from a fixed plane, they are limited to high magnification objectives, such as larger than 20xc3x97, which have a narrow depth of focus and short working distances, both of which can be a problem when inspecting generally large pieces of glass such as in the range of 550xc3x97650 mm to about 850xc3x97900 mm.
During the inspection of such relatively large sheets of glass, it is difficult if not impossible to hold such large pieces in a fixed plane, and often the variation of the glass surface across the sheet is larger than the glass thickness itself. Since such relatively large pieces of glass may have a thickness of only about 0.7 mm to about 1.1 mm, they have a tendency to sag and bow. Further, when inspecting liquid crystal display glass, if anything were to touch the surface of either the front or the back of such glass, it would create a defective piece, and accordingly such glass pieces must be handled only by edge portions thereof. To illustrate the problem of sag, if a piece of 850xc3x97900 mmxc3x970.7 mm glass were laid horizontally and supported only by the edges, the piece would sag in the center about 2 to 3 inches, whereas if the same piece were supported vertically, the sag would be much less, however, the piece would still bow somewhat in one direction or the other.
When utilizing a high magnification objective greater than 20xc3x97, many times it becomes virtually impossible to image the particles on both surfaces of a glass specimen because the working distance of such objectives is often less than the thickness of the glass sheet. Extra long working distance objectives (ELWDs) can help to alleviate some of these issues, but ELWDs are often more expensive than conventional objectives. Accordingly, the sheet must be turned over or reversed in order to image both the front and back surfaces, thus requiring a second set up.
It thus has been an object of the invention to provide a focusing filament technique which furnishes a solution to the problems set forth in the prior art, by providing an absolute surface reference that can automatically compensate for variations of the glass position-relative to a plane, and which also may be used with relatively low magnification objectives of about 10xc3x97 to 20xc3x97 that have a fairly large working distance of about approximately 7 to 10 mm.
The present invention sets forth a method and a structure for providing automatic focusing on a designated surface of a glass specimen through the utilization of a thin filament positioned in the light path of an inspection system. The filament is positioned such that it is focused to the same plane as the objective lens used to image a particle on the glass. A shadow of the filament can be seen in the field of view when the objective is focused on either the front or back surface of the glass, using bright field illumination. Since the shadow of the filament appears on either the front or back surface of the glass specimen, it provides an absolute surface reference that can automatically compensate for variations of the glass position relative to a plane.